Why does my armature's bone rotate 360°, but the mesh only rotates 180°?
Your head also inherits roatation from the torso bone. Look at the vertex groups: The head's vertices are members of both groups, 'Head' and 'Torso'. Thus the rotation of the head and the torso bones are combined by a weighted average. The weights of both groups are 1, so this results in equal influence of both bones. The torso-bone does not rotate, so the total rotation is halved.
Just remove the 'Torso' vertex group from the head and it rotates as intended.